Abstract
The atmospheric stable boundary layer (SBL) with a low-level jet is simulated experimentally using a thermally stratified wind tunnel. The turbulence structure and flow characteristics are investigated by simultaneous measurements of velocity and temperature fluctuations and by flow visualization. Attention is focused on the effect of strong wind shear due to a low-level jet on stratified boundary layers with strong stability. Occasional bursting of turbulence in the lower portion of the boundary layer can be found in the SBL with strong stability. This bursting originates aloft away from the surface and transports fluid with relatively low velocity and temperature upward and fluid with relatively high velocity and temperature downward. Furthermore, the relationship between the occurrence of turbulence bursting and the local gradient Richardson number (Ri) is investigated. The Ri becomes larger than the critical Ri, Ricr = 0.25, in quiescent periods. On the other hand, the Ri number becomes smaller than Ricr during bursting events.
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Abbreviations
- g :
-
Acceleration due to gravity
- Re δ :
-
Reynolds number based on δ (= U ∞ δ/ν)
- Ri δ :
-
Bulk Richardson number (\({= g\delta\Delta\Theta/\Theta_{o}U_{\infty}^{2}}\))
- Ri:
-
Local gradient Richardson number \({(=(g/\Theta)(\partial\Theta/\partial z)/(\partial U/\partial z)^{2})}\)
- U ∞ :
-
Freestream velocity
- U max :
-
Maximum value of mean streamwise velocity of LLJ
- U, W :
-
Mean velocity components in x and z directions
- u, w :
-
Fluctuating velocity components in x and z directions
- \({\overline{uw}}\) :
-
Vertical turbulent momentum flux
- \({\overline{u\theta}}, {\overline{w\theta}}\) :
-
Horizontal and vertical turbulent heat fluxes
- δ :
-
Boundary-layer thickness where U velocity shows a maximum value
- μ, υ :
-
Coefficients of dynamic and kinematic viscosity
- Θ:
-
Mean temperature
- Θ∞ :
-
Temperature of ambient air
- Θ s :
-
Temperature of cooled floor
- ΔΘ:
-
Temperature difference (= Θ∞ − Θ s )
- Θ o :
-
Average absolute temperature in boundary layer
- θ :
-
Fluctuating temperature
- σ u , σ w , σ θ :
-
Standard deviation of u, w and θ fluctuations
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Ohya, Y., Nakamura, R. & Uchida, T. Intermittent Bursting of Turbulence in a Stable Boundary Layer with Low-level Jet. Boundary-Layer Meteorol 126, 349–363 (2008). https://doi.org/10.1007/s10546-007-9245-y
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DOI: https://doi.org/10.1007/s10546-007-9245-y